[POWERPC] cleanup dma_mapping_ops

For pseries IOMMU bypass I want to be able to fall back to the regular
IOMMU ops. Do this by creating a dma_mapping_ops struct, and convert
the others while at it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>